An optical probing technology using near-field light has been noticed as a technology by which observation, processing or manipulation is realized in a micro-region with a size equal to or below a diffraction limit (500 nm or less, typically) (Japanese Patent Laid-Open No. HEI-10 (1998)-2905, Japanese Patent Laid-Open No. HEI-10 (1998)-132831, Japanese Patent Laid-Open No. 2000-329773, or, Japanese Patent Laid-Open No. 2001-13154). There are the following advantages in use of a sharpened optical fiber as a probe: (I) that an insulating material can be observed or processed because the fiber does not use electricity, different from the case of STM (scanning tunneling microscopy); (II) that a sample in a solution can be observed in the solution because light can propagate in water; (III) that energy states, optical properties and the like, can be simultaneously measured because spectrums and polarizations can be measured, different from the case of AFM (atomic force microscope); (IV) that the fiber can be used as manipulation such as a pair of optical tweezers by using a potential field caused by the near-field light; and (V) that the fiber can be used as lithography by using a photochemical reaction.
Recently, a trial, in which linearly polarized light is irradiated to or concentrated on the micro-region with a size equal to or below the diffraction limit by using a polarization maintaining optical fiber instead of a single-mode optical fiber which has been used so far, is beginning to be performed. However, the technology is still being developed and an optical fiber probe with a high transmission efficiency and a large polarization degree has not been realized yet for the time being.
It is well known that, as shown in FIG. 7, there are two kinds of optical fiber, that is, a PANDA type and an elliptical-core type as the polarization maintaining optical fiber. The polarization maintaining optical fiber of the PANDA type has a more complex structure, but is more easily manufactured, and has a larger polarization degree in comparison with the case of the polarization maintaining optical fiber of the elliptical-core type. Moreover, the polarization direction of the polarization maintaining optical fiber of the PANDA type can be confirmed by observing a section of the fiber with an optical microscope. On the other hand, it is difficult to confirm the polarization direction of the polarization maintaining optical fiber of the elliptical-core type. The reason is that it is impossible optically to observe a core of the fiber because the core has a similar size as that of the wavelength, or the core size is equal to or smaller than that of the wavelength, that is, is equal to or below the diffraction limit.
On the other hand, the polarization maintaining optical fiber of the elliptical-core type is preferably used when a probe is manufactured, using either of these two kinds of fibers. The reason is that two kinds of methods, a melting and pulling method and an etching method, may be used as a method for manufacturing the probe in the case of the polarization maintaining optical fiber of the elliptical-core type. Only the melting and pulling method can be applied for manufacturing the polarization maintaining optical fiber of the PANDA type. Here, “the melting and pulling method” is a method in which an optical fiber (82) is physically sharpened by heating and melting with heating means (81) such as a hydrogen flame, laser beams, and arc discharge, and by pulling the optical fiber (82), as shown in FIG. 8. “The etching method” is a method in which a clad region (92) and a core region (93) of the optical fiber are dissolved by using an etchant (91) such as hydrofluoric acid for etching, as shown in FIG. 9, and the core region (93) is chemically sharpened. A physical method such as the melting and pulling method has an advantage in simple processing, but a probe with a high transmission efficiency can be obtained by applying a chemical method such as the etching method.
In the polarization maintaining optical fiber of the PANDA type, it is impossible to form a probe because a stress-applying region remains undissolved when the fiber is chemically sharpened as shown in FIG. 10. The reason is that a material with high chemical resistance such as beryllium is doped in the stress-applying region.
As described above, the polarization maintaining optical fiber of the PANDA type has advantages that the fiber is easily manufactured, it has a large polarization degree, and the polarization direction can be confirmed by observing the section of the fiber with an optical microscope. On the other hand, since a chemical method cannot be applied for the polarization maintaining optical fiber of the PANDA type, it has been considered that it is difficult to manufacture a probe with a high transmission efficiency, using the fiber of this type. Then, development of a new method for forming a probe in an easy manner has been required.
Accordingly, the invention according to the application has been made, considering the above-described circumstances, to provide a new method for manufacturing an optical fiber probe with a high transmission efficiency and with a large polarization degree, and a new method for processing a micromaterial using the optical fiber probe.